The present invention relates to a liquid crystal display device, comprising at least one retardation film, a nematic liquid crystal layer, and a reflective cholesteric colour filter layer.
The market share of liquid crystal displays (LCD) is continuously increasing at the cost of other display technologies. This progress can be attributed to continuous improvements in size and performance and the decrease in the price of the panels. The design of LCDs for portable applications like cellular telephones and for demanding applications like PDAs differs strongly from the design of LCDs for high-end applications like monitors and TVs. For displays in telecom applications, low cost and low power are equally important as display image quality. Many challenges still remain to further improve the properties of LCDs. A performance issue that must be improved is, for example, the quality of the colours for reflective passive matrix displays, as initially described. Such display panels based on cholesteric colour filters are an attractive alternative for absorbing colour filters.
A passive matrix display with reflective colour filters in accordance with the prior art is described in the patent document U.S. Pat. No. 5,555,114. This document discloses a liquid crystal display including a cholesteric liquid crystal layer for selectively reflecting circularly polarised light having a specific wavelength. One embodiment of the inventive display basically contains a polariser, a quarter wave-plate, a liquid crystal layer, and a cholesteric colour filter layer, in sequence. The driven liquid crystal layer may be a layer of ferro-electric, twisted nematic or super twisted nematic liquid crystal.
However, this display, as described in the patent document U.S. Pat. No. 5,555,114, has the problem that for twisted nematic liquid crystal, a display of high contrast that cover the entire range of the visible spectrum cannot be obtained.
Consequently, the object of the present invention is to provide a display device with twisted nematic liquid crystals, overcoming the above-described problems with the prior art, yet maintaining a fairly simple structure.
These and other objects are achieved by a liquid crystal display device, comprising a polariser, at least one retardation film, a nematic liquid crystal layer being comprised in an electro-optical cell, and a, preferably cross-linked, light reflective cholesteric colour filter layer, for reflecting either essentially left-circular or right-circular polarised light, being characterised in that said liquid crystal layer is a super twisted nematic liquid crystal layer having a twist angle in the interval 180-270 degrees, and in that the summed retardation R of the retardation film or the retardation films and the liquid crystal layer is equal to approximately R=(3+2n)xcex/4, where n=0,1,2,3 . . . By utilising super twisted nematic liquid crystals in the display, a multiplexable display is achieved.
Moreover, said retardation film or retardation films are preferably arranged between a front polariser and said super twisted nematic liquid crystal layer.
Said retardation layer preferably comprises a wide-band quarter-wave plate. Suitably, said wide-band quarter-wave plate consists of a quarter wave plate in combination with a half wave plate, thereby enabling the use of standard, well-tested layer components. Suitably, the front polariser, the retardation film or retardation films, the liquid crystal layer and the reflective cholesteric colour filter layer are arranged on each other in sequence.
In accordance with a preferred embodiment of the invention, said device further comprises a further retardation layer which is a twisted nematic polymeric film, being arranged between the front substrate and the super twisted nematic layer, said twisted nematic polymeric layer and super twisted nematic layer have mutually opposite twist directions.
Further preferred embodiments of the invention are described in the remaining sub-claims.